[Libguestfs] [PATCH nbdkit 2/2] cow: Ignore flush/FUA requests.

[Eric Blake]

On 1/26/21 4:07 AM, Richard W.M. Jones wrote:
> There's no point flushing the overlay - a temporary file which has
> been deleted and is discarded as soon as nbdkit exits.  We can gain
> some performance by avoiding pointlessly flushing the overlay to disk.
> 
> I also changed can_fua to return NBDKIT_FUA_NATIVE so that
> NBDKIT_FLAG_FUA is actually being set, rather than nbdkit emulating it
> with a call to .flush.
> ---
>  filters/cow/blk.h |  3 ---
>  filters/cow/blk.c | 19 ++-----------------
>  filters/cow/cow.c | 19 +++++++------------
>  3 files changed, 9 insertions(+), 32 deletions(-)
> 

> +++ b/filters/cow/blk.c
> @@ -54,9 +54,8 @@
>   * When writing a block we unconditionally write the data to the
>   * temporary file, setting the bit in the bitmap.
>   *
> - * We allow the client to request FUA, and emulate it with a flush
> - * (arguably, since the write overlay is temporary, we could ignore
> - * FUA altogether).
> + * Since the overlay is a deleted temporary file, we can ignore FUA
> + * and flush commands.

The comment is correct, and the patch implements what the comment suggested.

LGTM

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org